Include a few XLSForms that users can learn from

This was suggested in https://github.com/opendatakit/docs/issues/20, but I'd like to make it more actionable. Here are two forms that I think should be in the docs.
* [x] [All Widgets](https://docs.google.com/spreadsheets/d/1af_Sl8A_L8_EULbhRLHVl8OclCfco09Hq2tqb9CslwQ/edit#gid=0) - Every widget that is supported by Collect
* [ ] [Form Design Tips](https://docs.google.com/spreadsheets/d/1bDqv28UWysf6bRCo7fuLqcIwIWTK52qbKbzGyyflCxg/edit#gid=400481115) - A few form design techniques to improve data quality

I think they should probably go under Form Building > XLSForm. Maybe as a Note? Or under Tips and Best Practices?

I think we should also remove the sample Excel file under Using the Application because it's not as complete as the All Widgets Form

Locate the documentation pages for Form Building > XLSForm and Using the Application, then inspect how linked XLSForms and sample files are currently presented. Confirm the Form Design Tips form is included, the All Widgets form is used as the complete example, and the outdated sample is removed; verify the rendered navigation and links.

Written by the indexing model from the issue text.